Sir Alex Ferguson has voiced reservations about the impact David Beckham will have for Tottenham Hotspur if he returns to English football on a two-month loan to the London club.

As Tottenham cleared space in their squad for Beckham by agreeing a deal for David Bentley to join Birmingham City on loan until May, the club were in advanced talks with the former England captain.

Harry Redknapp, the Spurs manager, said a deal for Beckham could be completed in advance of his team's FA Cup third-round tie against Charlton on Sunday – although the Guardian understands an agreement is not imminent. Ferguson believes there is a downside to the short-term nature of the proposed deal from Los Angeles Galaxy.

"The thing with David if he comes to England is that he will have to go back in March," the Manchester United manager said on Sirius XM. "It's a very short-term loan and we had the same situation with Henrik Larsson in 2007.

"We utilised those two months well and Henrik played a great part, but we knew we would miss him at the very time when we would have liked to have had him – at the tail-end of the season. But that was the arrangement, so we had to move on."

Ferguson never contemplated moving for Beckham because he believes his former player is on the wane. "Obviously anyone at 35 is looking at the twilight years of his career," he said. "I have players at United like Paul Scholes, Ryan Giggs and Gary Neville who grew up with David and are at the same level. They know they are coming towards the end of their careers and the horrible part of being a manager is seeing that. These players are eventually going to retire. David knows that better than anyone."

That is not a view shared by Redknapp, who has described signing Beckham as a "no-brainer" despite having never seen the midfielder play for Galaxy or knowing for sure his level of fitness – Beckham's last competitive game was on 15 November. "He's not a punch-drunk heavyweight champion who's washed up and skint," Redknapp said. "This is a boy who's in the prime of life. I think he could give the other players a lift and he can definitely deliver a ball. It's no good having Peter Crouch if you can't get the ball to him in the air. Beckham has the ability to do that.

"He will be a great asset. There is a good chance we will get David Beckham. It's just a case of sorting out the length of the loan and one or two bits and pieces. Our secretary told me this morning that it's going to happen. He could be here on Sunday. We are at home to Charlton and hopefully he will be there."

While discussions are progressing between both clubs, Galaxy are nervous about allowing Beckham to play elsewhere given he suffered a ruptured achilles tendon during a loan spell at Milan last season and it is believed that an agreement over the number of games he would play at Spurs is a stumbling block to the loan being finalised, as is the financing of his lucrative contract.

Should the deal eventually go through than Sven-Goran Eriksson, like Redknapp, believes Beckham could be a success on his return to these shores . "Knowing David he would never go to Tottenham if he didn't think he could still do it in the Premier League," said the Leicester and former England manager. "He would never accept it. He's a little bit too proud for that."

Even though a deal for Beckham has not been tied up, Bentley has been allowed to leave. Redknapp was keen to off-load the 26-year-old, a decision he claimed today had partly been forced on him by the club's supporters.

"I played him against Arsenal [in the Carling Cup in September]; he'd been injured, came back and did well. But the crowd was on him, they were waiting for him to make a mistake and it's not easy when you play like that. He was low after the game and I thought then that maybe its time for him to move on."

One player who has arrived at Spurs is Bongani Khumalo. The 23-year-old defender signed from the South African league champions Supersport United having scored for his country in their win over France at the World Cup.

Spurs are set to be heavily changed against Charlton on Sunday as Redknapp looks to give the bulk of his first-team a rest ahead of next Sunday's Premier League fixture with Manchester United.